1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device, more particularly, a semiconductor device that suppresses decrease of channel mobility and occurrence of punch-through and that can be manufactured efficiently, as well as a method for manufacturing such a semiconductor device.
2. Description of the Background Art
In recent years, in order to achieve high breakdown voltage, low loss, and the like in a semiconductor device, silicon carbide has been adopted as a material for a semiconductor device. Silicon carbide is a wide band gap semiconductor having a band gap larger than that of silicon, which has been conventionally widely used as a material for semiconductor devices. Hence, by adopting silicon carbide as a material for a semiconductor device, the semiconductor device can have a high breakdown voltage, reduced on-resistance, and the like. Further, the semiconductor device thus adopting silicon carbide as its material has characteristics less deteriorated even under a high temperature environment than those of a semiconductor device adopting silicon as its material, advantageously.
An exemplary semiconductor device employing silicon carbide as its material is a semiconductor device, such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which controls existence/non-existence of an inversion layer in an channel region in accordance with a predetermined threshold voltage so as to conduct and interrupt a current. For the MOSFET, in order to achieve further reduction of on-resistance, it has been considered to employ a trench type device structure characterized in that the channel region is formed along a wall surface of a trench, instead of a conventional planar type device structure, (for example, see Japanese Patent Laying-Open No. 7-326755, Japanese Patent Laying-Open No. 2008-235546, and Japanese Patent Laying-Open No. 8-70124), for example.
Here, in the conventional MOSFET, when a body region in which the inversion layer is to be formed is adapted to have a high impurity concentration, channel mobility is decreased. Hence, the impurity concentration of the body region needs to be not more than a certain value. However, when the body region is adapted to have a low impurity concentration, the thickness of the body region needs to be increased to suppress complete depletion (punch-through) in the body region. Hence, when the body region is formed by means of, for example, ion implantation, the number of times of performing ion implantation required is increased. This makes it difficult to efficiently manufacture the MOSFET. Thus, it is difficult to manufacture the conventional MOSFET efficiently while suppressing decrease of channel mobility and occurrence of punch-through, disadvantageously.